In the mid-fifties it became clear that the extracellular mineral phase of bone cannot be in equilibrium with the general extracellular fluids or the circulation in the body as a whole. Despite great advances in our knowledge of bone and bone metabolism, our understanding of this bone:blood disequilibrium remains primitive indeed. Yet detailed information on the problem is prerequisite to an understanding of calcium regulation as a whole and to a rational approach to such unsolved clinical problems as osteoporosis (to name just one). Only recently has our laboratory been able to provide unambiguous evidence that ion fuxes to and from the skeleton are controlled by a cellular membrane function. It is the purpose of this proposed research to employ model systems to identify morphologically the cellular elements involved in the control of the ion-fluxes and to elucidate the cellular and hormonal mechanisms responsible for the regulation of these fluxes. Calvaria have proven to be suitable as an experimental model system. Calvaria will be incubated with space-markers (mannitol, polyethylene glycol, iodinated proteins, ruthenium red, etc.) together with enzymes and other reagents to elucidate the chemical nature of the diffusion barrier. Selective inhibitors and unphysiological ion gradients will be imposed to clarify the cellular mechanisms involved in producing ion gradients. Frontal bones with intact, partially stripped and stripped membranes will be studied in Ussing chambers to isolate single membrane-unidrectional fluxes and factors which affect them. The biochemical studies will be coupled with morphological studies employing use of the electronmicroscope and electronmicroprobe.